Starting with the Ivy Bridge (3rd) generation, Intel switched to using thermal paste between the core and heat spreader instead of solder on socketed desktop processors. Presumably this was done as a cost savings measure.
This caused a marked increase in core temperatures and thermal throttling. Enthusiasts discovered that you could remove, or "delid", the heat spreader and replace the "toothpaste" with higher quality paste or liquid metal to drastically improve temperatures (15-20c) and improve overclocking headroom.
Edit: This event is commonly reflected on to showcase Intel's greed at a time where they dominated the market. It wasn't until the i9-9900k that Intel went back to soldering heatspreaders for consumer CPUs, at which point they were forced to because they were being challenged by AMD.