The large part for the Greek campaign taking as long as it did was the British supplying troops to prop up the Greeks against the Germans.
That's not true.
Until the British landed in Greece, Greece had already turned the tide in Albania and had the Italians retreating.
So if you consider the Greek campaign to actually encompass the entire Greek-Axis war, from October 1949 onwards, then the Greek fought most of the campaign alone.
Without a British involvement in Greece, one could expect the Greek Front in Northern Greece to indeed collapse faster. Furthermore less offensives would take place against the Italians, since the Greeks would not be able to strip off the defenses in Thrace and Macedonia that much as they did in OTL. Special care would have to be taken to cover the access towards Central and Southern Greek and more troops may be withheld in the rear, unlike in the OTL. In OTL the Greek Army was trapped in Macedonia and Thrace, being bypassed by the Germans who drove South through Yugoslavia. This did happen in part, because the Greeks had British reinforcements landing at their backs and thus had the ability to forward deploy their troops to make a stand at the border.
Without the British there, the Greeks may decide to let go of Macedonia and Thrace unlike they did in OTL, thus retrating faster to the South. This was a standard operations plan at that time (and even during the Cold War later on) in case of massive invasion from the North. The border with Bulgaria and Yugoslavia was simple too lengthy to be effectively defended and there is not much depth, the Greek Army can retreat to. So, we have the Greek Army in Eastern Macedonia and Thrace forward deploying some troops to undertake delaying actions on the border (like in OTL), but the rest of the troops would make a stand further South, perhaps not at Thermopylae, but at Tembi. Losing there they would retreat further to the South, trying to cover Athens. If the Greeks are smart, they will blow up the Isthmus bridges while retreating, thus making access to Peloponese impossible to the Axis in the short term.
Perhaps the British could detach some ships to the Aegean and manage to sealift a large portion of the retreating US Army from Peloponese to Crete.
This could have major butterflies, since more Greeks in Crete could very well make the German invasion very difficult.