Poor or reduced athletic performance in racehorses is a continual problem that equine practitioners must deal with at racetracks throughout the country.
Most of these animals are not ill in the conventional, sense of the word so routine physical and diagnostic techniques will probably not define the problem. Specialized examinations and a good history are very important.
History of new owners, trainers, managament or environment changes often give many clues. Racing records or other documentation can also help establish a problem. Diets and changes in feeding routines are often factors as well.

After taking a thorough history, a veterinarian must determine at the outset if in fact the animal has never performed well or if there was a reduction from previous racing performances. With the former, the owners expectations may be too high and all examination procedures will turn up nothing of any value. Discretion and a good consultative process is necesary here.

With the latter, a detailed examination of various systems must be performed in an orderly fashion. A CRC/Serum Chemistry(blood test) is probably a good way to start. Such information will tell your veterinarian how the horses body is responding to the challenges of being an athlete in terms of his oxygen carrying capacity and his handling of the stress of racing. All internal organs can be broadly screened using the serum chemistry results and if suspicious values are found then more testing will likly be required.
Metabolic problems (liver,kidney,etc.) and muscular conditions especially subclinical or low grade forms of myositis or tying up are readily diagnosed with serum chemistry tests are are common causes of poor performance.
Most horses respond well to proper training and conditiong methods and as a result, blood results are can be over emphasized. Blood tests are however an excellent tool when used serially (monthly e.g.) to determine which way your horse is responding to the rigors of racing.
A thorough physical examination at rest and after STRESSFUL exercise is a must. Jogging would not be considered stressful exercise. On the track in full gear at high speed or on a high speed treadmill may give us some clues as to why this horse is not performing up to expectations.
High speed treadmill exams are good for scoping while at speed also and for cinematography or videoing the whole event then playing it back later and trying to find abnormalities present.

Cardiac examination before and after work occasionaly determines a problem. Auscultation +/— an ECS are commonly used methods of determing potential heart problems. Some of the more common ones would be atrial fibrillation or certain pathological heart murmurs leading to poor performance.
Respiratory System examination is very important nowadays and in my experience this is where the bulk of poor performance problems originate from. Auscultation and Endoscopy(scope) are very diagnostic procedures that again need to be performed before and after fast work (at least a 2.15 mile). Scoping is even better still if performed within 1—4 hours after racing especially if exercise induced pulmonary hemmorhage (bleeding) is what you are trying to rule out.
Besides bleeders (exercise induced pulmonary hemorrahage/EIPH), which would be classed as lower respiratory problems, allergic bronchitis or COPD are also common. Nowadays horse with soft palate conditions, epiglottic entrapments and other upper respiratory conditions are being more and more diagnosed. As mentioned, high speed treadmill scoping may help differentiate palate abnormalities that do not show up at rest. History and rule outs of other conditions can also help pick out potential palate conditons.

A proper lameness examination is also an intrigual part of a poor performance exam. Often overlooked because the trainer says the horse is not lame, subtle lamenesses are probably the second most commen cause for poor racing. This time exams at slow and faster speeds are essential as well as a good discussion on how the horse has been driving and steering lately. Xrays +1— blocks are ways of finding problems here. Included in this category would be a visual exam of the neurological (nervous) system and any abnormalities that would require more specialized testing.

Poor or stale attitudes are often a cause of poor performance and again with trainer history and rule outs on a physical, hematological and endoscopic exams, we often arrive at this factor.
Stale or worn out attitudes are often easily fixed by turnouts and time off from the race track or often Just a change in training techniques (e.g. lots of Jogging and no training or more turn outs,etc.). Poor or “sour” attitudes are often not admitted by owners or trainers and they are more difficult to treat. Frequently, we turn to some type of hormonal therapy.

If I can not find any significant abnormalities in my work up, but from the history I am sure that there is in fact a reduced performance present, then frequently I try various treatments or procedures and see what we get for a treatment response.
No two horses and no two training programs are the same, so it is difficult to apply a standard testing protocal to all horses and come up with comparable results. Each horse,training, and conditioning program should be considered individualy and evaluated accordingly.