Are you absolutely sure that you have slowed down, and staying in that pace, just because you're out of touch for a while? You were able to clear the test(s), and you are also able to accomplish things, just not as per their expectations.
What I'd suggest is that, before second-guessing yourself (which will eventually take your morale further downwards), think what you can do to make things better. Every time you get any work, please don't jump-start, rather analyse the effort and time which you believe you'll need in order to complete the work. Plan it properly, so that execution phase takes less time by avoiding getting stuck / blocked. Check for any dependencies or blockers you may have and propose solutions (if you can) or ask for help.
In case you're given a timeline and your arrived timeline does not match, raise a flag immediately. Talk to your manager (or whoever is assigning you the work) so that they can also have their expectations aligned. The earlier you'll let them know about this in the process, the more are the chances that your proposed timeline is likely to be accepted (as long as you can justify your timeline if asked). If you stay silent and then do not deliver - it'll eventually create problems like you mentioned.
In case you're not given a timeline, or asked to come up with an estimate on your own, definitely let them know about your numbers and justify them.
Either way, you can arrive at a conclusion where you know how much to be done in what time and then you can start working.
Now, that being said, you definitely need to keep two things in mind:
You are not going to grow by doing what you had been doing. In case your best effort falls short of expectation, you need to learn new things / ways to get the job done faster, better and more reliable ways. Ask for help - learn a new tool, technology, automation - anything and everything you'll need in order to improve yourself.
Thinking about the problem and not doing anything about it is not going to make things better. Ideally, you should have been preparing yourself already for then job, even before you got one. The best time to start polishing up your skills was the time when you started applying, the next best time is NOW. Now you have the required resource and environment to get back in track and make it better, utilise it. Put extra efforts to cover up for the lags you think you are having and then push some more, be consistent and you'll be back.
Best of luck.