The best way to see if disk IO is the problem is to run iostat. I run "iostat -x 10" as I find that if a disk is at high load for 10 seconds that usually means that there is a serious performance issue. In the top output you can see that multiple users have IMAP processes blocked on disk IO which is an indication that disk IO speed is probably the issue.
The mail server VM iostat -x 10 Linux 2.6.32-573.12.1.el6.x86_64 (clients.webgen.com.au) 19/01/16 _x86_64_ (5 CPU) avg-cpu: %user %nice %system %iowait %steal %idle 1.56 0.01 0.58 4.68 0.04 93.14 Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util vda 1.20 14.03 1.51 4.86 60.27 145.11 32.21 0.21 33.21 6.05 3.86 vdb 0.08 19.10 10.89 0.84 982.21 161.89 97.52 0.27 22.99 6.96 8.17 vdc 0.30 8.25 18.70 11.90 1396.54 158.89 50.83 1.98 64.62 3.61 11.05 dm-0 0.00 0.00 1.35 17.22 49.29 137.51 10.06 0.19 10.34 1.72 3.20 dm-1 0.00 0.00 1.37 0.95 10.96 7.59 8.00 1.08 467.01 4.59 1.06 dm-2 0.00 0.00 19.03 20.11 1396.54 158.89 39.74 1.46 37.35 2.82 11.05 dm-3 0.00 0.00 10.97 20.24 982.21 161.89 36.66 0.98 31.25 2.62 8.17 avg-cpu: %user %nice %system %iowait %steal %idle 1.17 0.00 0.70 44.64 0.06 53.43 Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util vda 0.90 5.20 3.00 3.70 96.80 68.00 24.60 0.79 117.54 31.31 20.98 vdb 0.20 57.90 126.70 1.70 4590.40 475.20 39.45 2.77 21.47 7.72 99.17 vdc 0.00 3.20 68.90 15.80 1103.20 147.20 14.76 4.56 53.92 11.70 99.09 dm-0 0.00 0.00 1.70 8.40 78.40 67.20 14.42 1.59 157.81 19.95 20.15 dm-1 0.00 0.00 0.00 0.10 0.00 0.80 8.00 0.00 3.00 3.00 0.03 dm-2 0.00 0.00 68.70 18.80 1102.40 147.20 14.28 4.61 52.67 11.32 99.09 dm-3 0.00 0.00 126.80 59.40 4588.80 475.20 27.20 3.81 20.43 5.33 99.17 avg-cpu: %user %nice %system %iowait %steal %idle 0.92 0.00 0.56 41.01 0.04 57.46 Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util vda 0.00 7.00 0.00 2.60 0.00 72.00 27.69 0.14 55.31 43.92 11.42 vdb 0.00 2.20 101.20 1.10 5320.80 31.20 52.32 1.21 11.92 9.68 99.01 vdc 0.60 2.30 104.10 2.10 1779.20 33.60 17.07 1.33 12.49 9.39 99.75 dm-0 0.00 0.00 0.00 9.00 0.00 72.00 8.00 0.81 90.49 12.69 11.42 dm-1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 dm-2 0.00 0.00 104.70 4.20 1779.20 33.60 16.65 1.34 12.29 9.16 99.75 dm-3 0.00 0.00 101.20 3.90 5319.20 31.20 50.91 1.21 11.62 9.42 99.01