New timing but will barely get up to speed

What kind of mechanic did the work?
Which X and engine are you dealing with here?
You might try adjusting the distributor to see if the timing is off.

Satito said:

In addition to the above, since the belt broke it is entirely possible that one of the cam shafts had an extra rotation with the same result as stated above. It is easy to assume that because all of the marks line, the timing is correct, but the only way to really tell for sure that I know of is to take the valve covers off and check the valve positions as you rotate the crankshaft manually.

Click to expand...

I've only worked with my '00 3.3, but my camshaft gears have impressed marks on them that would clearly indicate whether the cam was out of sync, even 180 degrees. For mine, its way easier to get to the timing belt area than to pull the manifold to get into the valve covers. The OP hasn't yet indicated which X they're describing...

I can't imagine a reputable mechanic would do all this work and return the vehicle to the customer with such an obvious power issue!