performance decrease in case of activity chaining

It totally depends usage of chaining.
Think of it like waiting in line, if some activities involve complex calculations, database interactions, or there are just too many forms in a row, users might experience delays. This is because Appian holds onto resources for the entire chain, similar to how a long line at a store can slow down everyone. To avoid this, keep your activity chains short and simple, and consider using asynchronous execution (working on tasks in the background) for resource-intensive activities.

When you are using activity chaining the chained nodes would run with first priority. If unnecessary activity chaining is done then this would impact the process execution engines if multiple instances are being triggered or often launched. Unless until you have a huge number of nodes and too many subprocess nodes in your model, I don't think you should be worried about it.

When you use activity chaining, all the chained activities execute on a single engine. This means that the processing of these activities is given a higher priority in the processing queue. If you chain too many activities together, it can take processing resources away from other activities happening on the platform for longer period of time. As a result, users or systems may experience a decrease in performance.