Here we see Tim practising his arm stroke, which needs more of an extension on the arm entry, glide & catch phase and the right arm needs work to extend to enter the water going forwards and not in front of the head, which it tended to do when Tim got tired. This causes immediate resistance in the water, slowing the swimmer down and wasting energy.

Keeping the stroke technique together, even when tired or exhausted, is critical. It often helps to slow things down, bring yourself back to the stroke, focus and regain the technique at the expense of what may be seen as some speed.  The reality is you will make that speed back by maintaining a better stroke.  Each phase of the arm stroke, especially the entry-catch-glide elements will help to decrease your resistance in the water and conserve energy.