Macbook pro very slow after battery change

I have an Early 2011 Macbook Pro (2GHz i7, 8GB RAM, SSD storage, El Capitan) and the battery was wearing out, so I have replaced it with an after market battery. The battery life of the laptop is now good, but performance is very slow. I get an almost permanent beachball, the mouse cursor is very laggy, screen animations are very slow and there is about a 1 second delay every time I click a menu.

I have closed most of the programs and used the disk activity monitor to try to identify programs hogging the system resources, but the performance of this unit was fine before the battery change.

I read that an SMC reset might be necessary and when I tried this, it appeared to solve the problem, but the next time I cold booted, I experienced the same slow performance issues.

I have not tried refitting the original battery, but am now considering this, as the laptop is unusable in this condition and refitting it might demonstrate that the battery is the cause of the issue (but leave me with a very short working time without mains power).

Can anyone offer some advice please.

Thanks

Charles