with
c as(
select dts tc,evt ec,(select first 1 cast(x.dts as char(25))||cast(x.evt as char(1)) tx from t x where x.dts>t.dts) tx
from t
)
,d as(
select tc,ec,cast(left(tx,25) as timestamp) tx,cast(right(tx,1) as smallint) ex
from c
)
select tc,tx,ec,ex,  iif(ec < ex and cast(tx as date)-cast(tc as date)<=1, datediff(minute from tc to tx), null) was_here
from d
where ec=1
order by tc;

with
d as(
    select  dts tc
            ,evt ec
            ,lead(dts)over(order by dts) tx
            ,lead(evt)over(order by dts) ex
    from t
)
select tc,tx,ec,ex,  iif(ec < ex and cast(tx as date)-cast(tc as date)<=1, datediff(minute from tc to tx), null) was_here
from d
where ec=1

(select first 1 cast(x.dts as char(25))||cast(x.evt as char(1)) tx from t x where x.dts>t.dts)

(select cast(x.dts as char(25))||cast(x.evt as char(1)) tx from t x where x.dts>t.dts  order by x.dts rows 1 )

SQL> set stat on;
SQL> set explain on;
SQL> select count(*) from (select tc.x,(select first 1 x from t tx where tx.x>tc.x order by tx.x) tx from t tc);

Select Expression
    -> Singularity Check
        -> First N Records
            -> Filter
                -> Table "TX" Access By ID
                    -> Index "T_X" Scan
Select Expression
    -> Aggregate
        -> Union
            -> Table "TC" Full Scan

             15813251

Current memory = 2450745104
Delta memory = 544752
Max memory = 2450796560
Elapsed time= 562.68 sec
Cpu = 0.05 sec
Buffers = 524288
Reads = 215467
Writes = 0
Fetches = 924779969

SQL> select count(*) from (select tc.x,lead(x)over(order by x) tx from t tc);

Select Expression
    -> Aggregate
        -> Window
            -> Record Buffer
                -> Table "TC" Full Scan

             15813251

Current memory = 2450734048
Delta memory = -11056
Max memory = 2536880256
Elapsed time= 122.29 sec
Cpu = 0.00 sec
Buffers = 524288
Reads = 0
Writes = 0
Fetches = 32017389
SQL> set explain off;
SQL> set stat off;