WordNet sense 1 (made heavy or weighted down with weariness):

weighted

WordNet sense 2 (marked by great psychological weight; weighted down especially with sadness or troubles or weariness):

heavy

WordNet sense 3 ((of movement) slow and laborious):

plodding

WordNet sense 4 (requiring great physical effort):

effortful

WordNet sense 5 (lacking in liveliness or animation):

dull

WordNet sense 6 (darkened with overcast):

gray, grey

WordNet sense 7 (full of or covered with clouds):

cloudy