Why is my MacBook running slow with external monitors .

I don’t know if this will answer your question, but running external monitors can be a drain on computer resources. When I have two monitors plugged into my 2018 13” MacBook Pro, I notice that Lightroom tends to run slower. (Until I bought an eGPU to run the external monitors and take the burden off the integrated graphics card.) Is the monitor you are using a newer, higher resolution monitor? It’s possible it just can’t handle that resolution at that refresh rate. Although unless it is 4k or greater, I’d think even a 2008 MacBook Pro could handle it. You can try to reduce the burden by running in clamshell mode (MacBook Pro screen closed).

I have also had issues with older Macs not always starting up when too many peripherals are connected. I don’t know why this happens, but you are not alone in experiencing that.