Image on mobile/ speed

Hi,

I’m using the Nisarg theme on my website https://www.boho-marketing.co.uk. I have customised my homepage but other pages (about,contact, etc.) still use the theme. When these pages are resized for tablet and mobile the header image seems to cut off.

When you shrink the browser size you can see the header image jump. Space appears between the end of the image and the line where the image should stretch to.

Can anyone help here?

Also, my site is incredibly slow. I’m aware this may not be a theme issue but any support would be greatly appreciated.

Thanks in advance.