Background: The fusion of heavy nuclei can be conceptually divided into two steps - capture of the projectile and target, then evolution towards a compact fully equilibrated compound nucleus. Reseparation through quasifission after capture inhibits compound nucleus formation, leading to fusion suppression. Purpose: To determine quantitatively the correlation between the degree of fusion suppression resulting from quasifission and the observed characteristics of the quasifission events themselves. Method: Evaporation residues have been measured for 28Si+192Os and 34S+186W forming the compound nucleus 220Th, and identified using implantation-decay. Fusion suppression factors for these reactions (and others from the literature also forming 220Th) were determined relative to the 16O+204Pb reaction. Fission characteristics were also measured for these three reactions, as well as for 48Ca+172Yb. Results: Overall, fusion probabilities are found to fall rapidly as the charge product ZpZt increases above the value for 16O+204Pb. Crucially, although significant suppression of the xn evaporation residue yield is observed for the 28Si and 34S reactions, there is no significant fission mass-angle correlation that would signal the presence of fast quasifission. However, fission mass widths are larger than those expected based on the 16O+204Pb mass widths, taken as corresponding to fusion-fission. Conclusions: Formation of an equilibrated compact compound nucleus can be suppressed by over 50% due to (slow) quasifission, even when no significant evidence of a mass-angle correlation is seen - often taken as evidence for the presence of quasifission. Evidence of slow quasifission is shown by the increased mass widths observed.