The standard ADSL provides between 5-10MB download and (theoretically) 800 KB upload. In practice the upload is often slower, and the upload is what effects website response time the most. If your using cable or fibre your upload speeds will be in excess of 10MB/S which plenty fast enough for a small website.
If your running a website or blog for personal use on ADSL, we have found the 500KB/s upload is satisfactory for 10 out of 30 days in a month. Some pages (especially those using a log of plugins) may at times take a little long to load (1-2 seconds to respond, but 10 seconds to finish loading all the images, scripts, Facebook pixels, Google Tags) so there are speed issues. However looking at Google Analytics for a site running from ADSL, I see this slow performance as an infrequent even for a small blog site receiving less that 500 visits a day, and those visitors inconvenienced by slow response are few and far between (e.g. 1or 2 a week may abandon (or Cntrl-F5 and try again) due to upload speed.
So perhaps optimizing front page load speed might be cheaper than upgrading to cable ?
I know this is not a complete answer but I hope it helps with your decisions.