Your plan running slow may be due to poor source quality or strict filters.
Another reason might be that the bot has run out of accounts to follow in your sources. A clear sign of this is when your plan ran at a decent speed for a time, then suddenly dramatically slowed down.
Before the bot performs follows it checks the account it is about to follow whether it passes your filters, then it decides to skip it or not. This usually takes split seconds to be done.
Slow running plans are often due to most accounts in your source not passing your filters. The split seconds it takes the bot to skip these accounts add up and slow your plan down.
Usually the filters that slow plans down the most are: Low max. followers/followings filter or skipping private accounts.
Please try to adjust your filters and/or swap your sources to get a desired follow speed. If you think your source may have no accounts left to follow, try changing it. Alternatively speed up your plan to get the desired follow speed.