It's hard to give generalized advice. There are many reasons why development is slow and it's only partly up to you. Getting packed with too many responsibilities can easily make you slow especially if you must also do typical operations/PM/junior onboarding. Sometimes refusing more work is the best thing to do and sometimes due to turnover it's just better to leave.
Since you've mentioned fullstack I guess this means you decided not to specialize. Being a little bit of everything is useful for some types of companies and work but is one source of also being inefficient. It's easier to pick frontend, backend, ops or some other specialization. Backend development is a bit more stable than FE but it depends on your taste. Having less specialities means its also easier to fit into certain job postings and crunch through the interviews but it seems to me you don't have an issue there. Being more specialized also passively makes you more efficient in that domain.
Also note that by switching jobs per year you've probably spent half of that time being onboarded. Obviously you'll be slower during the first few months at works unless you're doing something very standard and well defined. I'd also question the underperforming part. Is this due to overpromising? I've regularly worked with devs who overpromised and underdelivered. The trick here is also to properly time those estimates and take on as much work as you can carry. Note that when a company wants to get rid of you they'll always say its a competitive market or claim underdelivery but the real reasons are much more varied.
Since you've mentioned fullstack I guess this means you decided not to specialize. Being a little bit of everything is useful for some types of companies and work but is one source of also being inefficient. It's easier to pick frontend, backend, ops or some other specialization. Backend development is a bit more stable than FE but it depends on your taste. Having less specialities means its also easier to fit into certain job postings and crunch through the interviews but it seems to me you don't have an issue there. Being more specialized also passively makes you more efficient in that domain.
Also note that by switching jobs per year you've probably spent half of that time being onboarded. Obviously you'll be slower during the first few months at works unless you're doing something very standard and well defined. I'd also question the underperforming part. Is this due to overpromising? I've regularly worked with devs who overpromised and underdelivered. The trick here is also to properly time those estimates and take on as much work as you can carry. Note that when a company wants to get rid of you they'll always say its a competitive market or claim underdelivery but the real reasons are much more varied.